Digital Multimeters

When problems occur in your boat’s electrical system, the first tool you should reach for is the multimeter. By definition, the multimeter is an electronic measuring instrument that combines several metering functions into a single case design. Thus a basic multimeter usually consists of an ammeter, voltmeter, and ohm-meter, with each “meter” being selectable from a dial on the front of the meter case and viewed on a common display. More advanced multimeters can measure parameters such as capacitance, inductance, temperature, and even include test sockets for checking the health of diodes and transistors. The very high-end multimeters are usually classified as industrial or scientific meters. These top-tier units will interface with a PC and can even display a graph of waveform shape, much like an oscilloscope, right on the meter’s LCD display. 

The first truly portable multimeters saw service in WWII during the mid-1940s. These early “Volt Ohm Meters” (VOM) were primarily manufactured by the Simpson Company, and featured internal battery packs and analog metering. In the mid 1950s, Hewlett Packard entered the field with their “VTVM” (Vacuum Tube Voltmeter). HP’s VTVM was powered by AC (designating it primarily as a bench-test meter), contained vacuum tubes (as its name applies) and featured the same D’Arsonval analog meter movement as the Simpson VOM. The VTVM multimeter was considered the premier meter of its time. In the 1960’s Heathkit offered a VOM “Kit” whereby the purchaser actually assembled and soldered together his own analog meter.

All of these early multimeter designs were deficient in meter accuracy. This was because all analog D’Arsonval physical meter movements are inaccurate when measuring readings that are not at least mid-scale on the selected meter range. Another drawback of the early meter designs was that the meter needed to be calibrated to zero before an accurate Ohm measurement could be obtained.

Once LCD (Liquid Crystal Display) technology took hold in the ’80s, the multimeter’s new acronym became the DMM (Digital Multimeter). You can almost date how long a technician has been in the industry by the acronym he gives to his multimeter.

As technology moved forward and most analog VOMs gave way to DMMs, the case size of the DMMs decreased, measurement accuracy increased significantly, and additional measurement features were added. Today you can purchase pocket-sized DMMs (smaller than a deck of cards) in colors that coordinate with your toolbox. But beware, there are scores of inexpensive private-label DMMs on the market coming from Pacific Rim plants that are downright junk. There’s nothing more frustrating than trying to solve an onboard electrical problem when you can’t rely on the readings that your DMM is displaying.

What the Pros Use
Selecting a test group of basic durable meters that would be worthy of the mariner’s toolbox was a daunting task. We asked the pros at a number of top New England Boat Yards what their DMM preferences were. What we found was that the pro’s meter of choice was as varied as the acronyms that they assigned to their meter. The marina mechanics and riggers tended to prefer products from Ideal Industries and Triplett, while the technicians opted for multimeters almost exclusively from Fluke. There was no single “popular” model used by either group surveyed—everyone had an affinity towards their own personal meter. The consensus was that once a quality meter was purchased, it was expected that the meter would remain in service for a good many years, and that the readings observed on the meter were absolute.

Must-Have Features
• Auto Power Off. If a meter doesn’t automatically turn itself off after a pre-set time, avoid it. You definitely don’t want to start out on your electrical trouble-shooting mission with a dead battery in your meter.

• Low Battery Indicator. It’s important to know the health of your meter’s battery to avoid questioning the validity of the readings you measure.

• Overload Protection. This feature protects the meter from damage if the dial selector switch is set for the wrong parameter test; e.g. the meter is set to measure resistance (ohms) and you inadvertently measure an AC voltage. On non-auto ranging modes, the overload protection circuit will protect from over-voltage conditions if the metering scale was set too low. The best type of overload protection is one that provides protection on all ranges. Also, pay attention to what kind of protection the meter offers in the current measuring mode. Most quality meters are either fuse-protected or thermally protected in the current test at 10-amps. The majority of the low-cost DMMs do not provide any current overload protection. Therefore, if you try to measure a current draw above 10 amps (which is not a lot of current when searching out a problem) with an unprotected DMM, be prepared afterwards to open the meter’s case to let the smoke out. Game over.

• Continuity Check. When this feature is selected, the meter emits an audible tone when the two meter probes are shorted together (continuity). Thus one is able to “ring” out a circuit and verify the presence or absence of continuity without having to view the meter.

• Data Hold. When working in a tight or dimly lit area (most of the work areas on a boat) the data hold feature will allow you to press a button on the meter to retain the reading on the meter until you can shed some light on the meter’s LCD.

• Max/Min Hold. This feature is very helpful when measuring voltage pull-down on a circuit when certain motors or appliances are first turned on. If the voltage reading on the circuit falls to a lower reading than when the motor was turned on, the meter automatically stores the lower reading.

• Auto Ranging. An auto-ranging DMM does not require you to set the range scale on the meter’s dial. The DMM will automatically sample the input and choose the range scale that yields the best resolution. You will still have to select the correct mode (DC/AC Volts, Amps, etc.).

• Quality Electrical Test Leads. Many of the meters that we have omitted from this comparison simply did not come equipped with quality insulated test leads. Our choice for insulated test probes are ones manufactured with very flexible silicone leads of at least 24″ in length. All too often, we found ourselves trying to untangle stiff lead sets on the less expensive models, only to have the leads disconnect from the meter and ultimately subject the meter prematurely to a drop test.

Also, look at the tips of the test lead set. Test lead sets with stainless steel tips make probing into electrical tie points more positive. With the stainless tips you are able to apply more pressure to the probe and press deep into a junction without fear of bending off the tip, as is often the case with a cheaper tin/steel-tipped set.

If you have a good DMM that you are partial to, but your test lead set is worn out or stiff, try ordering a replacement silicone set from companies like Meuller that specialize in test lead replacement kits.

How We Tested
We tested a sample cross-section of digital multimeters ranging in price from $30 to $130 (average street cost) that are readily available from national marine electronics chain stores, electrical supply houses, or via the Internet. Each meter was then put through a battery of accuracy testing, consisting of: measuring DC voltages from a Sorensen bench power supply set at 2.5, 5, 10, 20 and 40V DC; measuring AC voltage and frequency from a pure sine-wave inverter; measuring in-line DC current draw from a dry-running bilge pump motor, and measuring Ohm values from three precision-level resistors.

Accuracy errors were recorded, and each meter was then assigned a numerical score that combined separate, subjective ratings for user interface and ergonomics. (The higher the number, the better. See chart.)

The Meters
• Radio Shack RS22813. A decent compact meter, considering its $30 price. Features include auto range, auto off, overload indication, and a current test. The current test is limited to a maximum of 10 amps, but is non-fuse protected. The 22-813’s LCD display is sharp, with a good viewing angle. The meter does not have onboard test probe storage, nor does it have any sort of tabletop stand. The warranty is only for 90 days.

• Gardner Bender GDT185A. This pocket-sized DMM has none of the advanced features of the similarly priced Radio Shack model. We included it in our line-up because it’s stocked at many marine hardware stores and because it’s packaged with a thin folding carry case. The best application that we could think of for this meter would be as a travel meter, packed in the bottom of your duffel, to be used when a simple meter is better than no meter at all.

• Gardner Bender GDT200A. A compact DMM, a few dollars more than the RS 22-813. It includes a built-in transistor tester and an external temperature probe, but has no auto range feature and the 10-amp current test is unprotected. The LCD display is smaller than the Radio Shack model. This meter has acceptable accuracy, but no tabletop stand and one year of warranty.

• Ancor 702073 & 702078. These are modestly priced, full-size meters with fused circuits on the 10-amp current test. Both meters feature auto off, low battery alert, integral tabletop stand, onboard storage of test lead probes, and integral transistor tester. The 702073 is not an auto-ranging meter, while the -78 is. Ancor wraps each meter in a red protective rubber holster. The holster is nice, but increases the width of the meter to around 4″, which makes it a bit awkward to hold with one hand. Because the meter is large and heavy, Ancor should include a carry strap so that a user could hang or cleat-off the meter when working in tight spaces. This meter has a five-year warranty.

We learned that Ancor’s auto-ranging DMM is in fact built by Gardner Bender. GB has an accomplished history of producing quality test meters. Save a few dollars and gain a longer warranty with the Ancor.

• Triplett 9025. An entry-level pro-grade auto-ranging DMM, this meter has the same features and similar layout as the Ancor 702078, right down to the red protective boot. Triplett has a reputation for manufacturing very accurate and durable meters, and the 9025 measures up to that standard. Except for the three-year warranty, it would seem that the Triplett, with the pro’s stamp of approval, would be worth the extra few dollars over the Ancor 702078.

• Fluke 111. This is a new model, drop-protected to one meter and with the best accuracy rating of most sub-$400 DMMs on all waveforms. Everything about this full-feature meter is high-end, from the fused 10-amp current test circuit (which can handle 20 amps for up to 30 seconds), to the industrial yellow rubber case, to the rotary selector switch which glides positively between functions. This is definitely a technician-grade meter. When we showed this meter to the marina pros, almost everyone guessed that the unit was two or three times more expensive than it really is. This is an example of technology driving down prices without diluting quality and accuracy, and without the addition of superfluous features that are sometimes used to mask deficiencies in quality. If you pop a Fluke 111 out of your tool kit, everyone will know that you mean business.

Conclusions
The $30 Radio Shack DMM holds its own on the entry-level side of the field, while the Fluke 111 gets top honors for best in class under $200. At first glance, the Fluke may seem a little excessive for the average do-it-yourselfer. However, when you factor in what it costs to have a technician visit your vessel for just one hour, the meter will pay for itself—provided, of course, that you’re skilled or lucky enough to solve the problem.

Street prices for the Fluke range between about $110 and $140, but we’ve seen prices on e-Bay for “new” units at much lower prices. It’s hard to know what’s going on when there’s a price differential like that, but we took a chance and bought one on e-Bay for $75. It was a deal that even PS couldn’t pass up.

Also With This Article
“Value Guide: Digitial Multimeters”
“Wire Tracker”
“Tips from the Pros”

Contacts
• Ancor Products, 800/424WIRE, www.ancorproducts.com
• Fluke Corporation, 800/44FLUKE, www.fluke.com
• Gardner Bender, 800/822-9220, www.gardnerbender.com
• Radio Shack, 800/THESHACK, www.radioshack.com
• Triplett Corp., 800/TRIPLET, www.triplett.com